Thermal imaging, or thermography, is a recording process wherein images are generated by the use of image-wise modulated thermal energy. There are two commonly known methods for thermal imaging. The first is generally referred to as thermal dye transfer printing and the second as direct thermal printing. Thermal dye transfer printing generally involves heating a donor element to transfer dye from the donor element to a print media to produce a desired image. Direct thermal printing involves directly heating a thermosensitive imaging media to cause a chemical reaction that produces a desired image on the imaging media.
In direct thermal printing, application of heat to the imaging media is generally accomplished through use of a thermal recording head or printhead. Thermal printheads typically comprise a number of microscopic heating elements, generally resistors, spaced in a line-wise fashion across the printhead. The thermal printhead prints one line of pixels of an image at a time, with each resistor producing one pixel of the line of pixels on the thermosensitive media. A media transport system (e.g. a dc stepper motor) incrementally moves the imaging media past the thermal printhead as the individual lines of pixels are printed such that the desired image is constructed from a large number of individually printed lines of pixels.
The printed density produced by each heating element is a function of the amount of thermal energy transferred by the heating element to the imaging media, with a greater amount of thermal energy producing a higher printed density. Conventionally, the heating elements are binary controllable devices, meaning that only on/off control is employed to control the amount of thermal energy generated and transferred to the print media by the heating element.
When printing a desired image, an image data value for each pixel is received, with each data value being representative of a desired printing density. The image data value for each pixel is converted into a corresponding series of 1-bit values and a time-multiplexing scheme is employed to consecutively feed the 1-bit values of each series to the corresponding heating element. This process is commonly referred to as “time slicing.” To print each pixel, each 1-bit value of each series is transmitted to the corresponding heating element for a same duration, commonly referred to as the time step, or time slice, of the slicing process.
A strobe signal is employed to enable each heating element to become energized during each time step so as to generate thermal energy based on the state of the corresponding 1-bit value of the series. To provide different printing densities, or “gradation levels”, the sequences of 1-bit values are arranged so as to energize the heating element for a time necessary to produce a desired printing density. The arrangement of each sequence can be based on several factors, such as a temperature of the corresponding thermal element, for example, such that different sequences of 1-bit values may be provided to different thermal elements to produce a same printing density.
The resolution of a thermal printing apparatus depends on the number of gradation levels that can be generated by the printhead. The higher the number of gradation levels, the higher the resolution and the higher the quality of the printed image. In order to increase the resolution, several approaches have been employed to increase the number of gradation levels capable of being produced by the printhead.
According to one type of conventional thermal printing apparatus, a sequence of N 1-bit data values must be provided to each thermal element to provide N gradation levels. One approach employed to increase the number of gradation levels is to simply increase the number of 1-bit data values provided to each thermal element. However, increasing the number of gray levels in this fashion increases the time required to print each line of pixels. To maintain print times at acceptable levels requires an increase in the clock frequency and/or an increase in the degree of parallelism of the printhead (e.g. more shift registers). However, increasing the degree of parallelism increases the complexity and cost of the printhead and the clock frequency is limited by the printhead components (e.g. shift-register operating frequencies).
Other approaches to increase the number of gradation levels involve varying the pulse width of the strobe signal (i.e. the enable time of the thermal element). Such approaches generally increase the number of available gradation levels without increasing the required clock frequency. However, with some approaches, varying the strobe pulse width in this manner results in the thermal elements being heated in a non-continuous fashion. Thus, even though the thermal elements may be energized for an amount of time corresponding to a given gradation level, heat dissipation of the thermal elements in the time interval during which they are not energized may result in a lower than desired print density.
In another approach, heating of the thermal elements is substantially continuous when varying the strobe pulse width, but is centered within the corresponding pixel area on the imaging media as it is moved past the thermal element during the printing of the pixel data. While such an approach may be desirable for halftone image recording, it provides poor uniformity of the print density across the pixel area and is not desirable for continuous tone imaging, such as employed for medical imaging purposes, for example.
It is evident that there is a need for increasing the gradation levels available without increasing required clock times, particularly for thermal imaging systems configured to perform continuous tone thermal recording.